By using a nucleic acid probe such as an RNA probe or DNA probe that has been subjected to some form of labeling to detect and visualize the expression pattern of DNA or RNA at a cellular level, a multitude of problems related to vital phenomena can be explained. The labeling methods employed for the probes used in these cases can be broadly classified into “radioactive isotope labeling methods”, “fluorescent labeling methods” and “enzyme labeling methods.” Historically, nucleic acid probes having an introduced radioactive isotope were developed first, but in recent years, restrictions have been introduced relating to the handling of such probes, and therefore fluorescent labeling methods and enzyme labeling methods, which do not require the use of a radioactive isotope element, are attracting much attention.
In recent years, aptamers having binding specificity to organic small molecules or biopolymers such as proteins are attracting attention as detection ligands. Nucleic acid aptamers are oligonucleotides (DNA or RNA) that able to bind wide variety of target substances with high affinity and specificity by their higher order structure.
A chemical binding method is usually used as the method for introducing a labeling moiety such as an enzyme into a nucleic acid such as a nucleic acid aptamer. However, the chemical reaction may sometimes cause a deterioration in the target substance recognition ability of the nucleic acid aptamer or the like, or a deterioration in the performance of the labeling moiety. Particularly in those cases where the labeling moiety is an enzyme, the performance of the enzyme is prone to deterioration. Accordingly, there is much demand for a method for modifying a nucleic acid such as a nucleic acid aptamer while maintaining both the recognition ability of the nucleic acid aptamer or the like and the function of the labeling moiety. In other words, in order to maintain the functions of both the nucleic acid aptamer or the like and the labeling moiety, a modification method that strongly suppresses any effects on the target substance recognition site of the nucleic acid aptamer or the like, and on other active sites such as the labeling enzyme, namely a site-specific modification method, is very desirable.
On the other hand, a method is known in which a transglutaminase (TGase) is used to achieve site-specific binding of an exogenous molecule which is anionic and has a glutamine (Gln) residue that is recognizable by TGase to a peptide or protein having a lysine (Lys) residue or a primary amine that are recognizable by TGase (for example, see Patent Document 1).